1. Field of the Invention
The invention relates to a control process for a voltage transformer with a wide input voltage range and the application of such a process.
Transformers, e.g. isolating transformers, step-up transformers or step-up/step-down transformers, can be used to improve the power factor of power rectification. Such transformers are theoretically capable of continuously transforming an input voltage and supplying a regulated output voltage. With suitable control of such a transformer, it is possible, in connection with a power rectifier, to take a sine-wave current from an alternating current power supply and supply a regulated direct current voltage at the output. Step-up transformers with such characteristics are disclosed, for example, in the following articles or application reports by Michael Herfurth, one of the inventors of the instant application:
"Aktive Oberwellenfilterung fur Netzgleichrichter hoherer Ausgangsleistung" [Active Harmonic Filtering for High Output Power Rectifiers], Siemens Components 24 (1986), Volume 1, pages 9 to 13;
"TDA 4814--Integrierte Steuerschaltung fur sinusformige Netzstromaufnahme" [TDA 4814--Integrated Control Circuit for Sine-Wave Line Current Input], Siemens Components 24 (1986), Volume 3, pages 103-107;
"Aktive Oberschwingungsfilter mit TDA 4814 A fur 250 W Ausgangsleistung" [Active Harmonic filter with TDA 4814 A for 250 W Output Power], Siemens-Bericht LE8803, Aug. 19, 1988;
"Steuerschaltung mit TDA 4816 und TDA 4919 fur ein aktives Oberschwingungsfilter (Control Circuit with TDA 4816 and TDA 4919 for an Active Harmonic Filter], Siemens HL-Anwendungsbericht PD22 8905, November 1989; and
"Aktives Oberschwingungsfilter mit konstanter Betriebsfrequenz und 600 W Ausgangsleistung" [Active Harmonic Filter with Constant Operating Frequency and 600 W output], Siemens HL-Anwendungsbericht PD22 9002, February 1990.
The control principle of that circuit of the prior art requires a control amplifier, having an actual-value input to which a signal is applied that is a function of the output voltage of the transformer, which is compared to a command or setpoint value of the output voltage, and having an output that forms one input of a multiplier. Another input of the multiplier is a time-dependent analog signal which is proportional to the rectified input voltage of the voltage transformer. Since, in the steady state, the control amplifier supplies a direct current voltage as the output value, the multiplier supplies a signal at its output having a time-dependent curve which is defined by the rectified input voltage of the voltage transformer, and has an amplitude that is defined by the output of the control amplifier. The output signal from the multiplier forms a command or setpoint value entry or input for the input current of the transformer, i.e. the line current input. A circuit connected downstream of the multiplier supplies supplies control pulses for the power breaker or breakers of the transformer, as a function of the command or setpoint value input of the transformer input current and as a function of the actual curve of the transformer input current. The layout of that circuit is primarily a function of the type of transformer. One simple possibility for the activation of a switched-mode step-up transformer is disclosed in the above-referenced article "TDA 4814--Integrated Control Circuit for Sine-Wave Line Current Input", in which only one comparator compares a voltage proportional to the input current of the transformer to the command or setpoint value entry of the multiplier, and when a peak value is reached which corresponds to the command or setpoint value, deactivates the power breaker of the transformer, and whereby the power breaker is reactivated as soon as a zero current detector determines that the input current of the transformer has become zero.
An additional possibility for converting the command or setpoint value input of a multiplier output of a control circuit as described above into control pulses for power breakers of a transformer can be seen in the fact that downstream of the multiplier, there is an additional controller, which adjusts the actual value of the input current of the transformer to the command or setpoint value specified by the multiplier, and provides either an analog signal or a clocked signal supplied by a pulse width modulator for the activation of the power breaker or breakers of a transformer. A configuration which operates according to such a process is described, among other places, in the article "Active Harmonic Filtering for High Output Power Rectifiers" by M. Herfurth, reprinted from an article which appeared in "Siemens Components" 24 (1986), Volume 1, pages 9 to 13, and in the Siemens HL-Anwendungsbericht PD22 8905 under the title "Control Circuit with TDA 4816 and TDA 4919 for an Active Harmonic Filter", by M. Herfurth.
A common feature of all of the voltage transformers described above is a control process with a multiplier, having an output signal which provides the reference value for a current comparator or current controller, whereby the current comparator or current controller controls the input current of the voltage transformer. The input voltage of the transformer, which has an amplitude that has been reduced by a voltage divider, is applied to one of the inputs of the multiplier, and the output voltage from a voltage regulator which controls the output voltage of the transformer is applied to the other input of the multiplier. Thus a signal is provided at the output of the multiplier which has the wave form of the rectified line voltage, and the amplitude of which can be modified by the voltage regulator.
Since the transformers described above can transform continuously and with high efficiency to another voltage level, it can be inferred that such transformers can also be used to feed a load both from a 110 V power supply or main and also from a 220 V power supply or main, without the need for additional switching measures. That is possible with the transformers described above, which operate using the control processes described above.
Due to their circuitry, multiplier circuits and their signal inputs have a restricted dynamic range. In order to get a defined output level on the multiplier, the input level cannot drop below a specified value at any of the signal inputs of the multiplier.
Moreover, the input signal level cannot exceed an input-specific value at any of the inputs, so that the output signal of the multiplier circuit is a function of the input signal, in the desired manner. Therefore, in voltage transformer circuits in which the above-mentioned control process is realized, this restricted dynamic range of a multiplier circuit also restricts the allowable input voltage range, or the quotient of the maximum allowable input voltage and the minimum allowable input voltage. In order to provide a desired command or setpoint value at the output of the multiplier, namely when there is a fixed maximum allowable input voltage, the input voltage of the voltage transformer may not be so low that the input signal of the multiplier derived from it is below a specified level. Moreover, the input voltage of the voltage transformer may only be so low that to supply a desired command or setpoint value level at the output of the multiplier, the voltage supplied by the voltage regulator at the input of the multiplier does not exceed an allowable level.